Image sensors typically include a one-dimensional linear array or a two-dimensional array of light sensitive regions (often referred to as “pixels”) that generate electrical signals that are proportional to the intensity of the light respectively received in the light sensitive regions. Solid-state image sensors are used in a wide variety of different applications, including digital still cameras, digital video cameras, machine vision systems, robotics, guidance and navigation applications, and automotive applications.
One class of image sensors is based on charge-coupled device (CCD) technology. In a common implementation, a CCD image sensor includes an array of closely spaced metal-oxide-semiconductor (MOS) diodes. In operation, a sequence of clock pulses is applied to the MOS diodes to transfer charge across the imaging area. Another class of image sensors is based on active pixels sensor (APS) technology. Each pixel of an APS image sensor includes a light sensitive region and sensing circuitry. The sensing circuitry includes an active transistor that amplifies and buffers the electrical signals generated by the associated light sensitive region. In a common implementation, APS image sensors are made using standard complementary metal-oxide-semiconductor (CMOS) processes, allowing such image sensors to be readily integrated with standard analog and digital integrated circuits.
An individual image sensor chip oftentimes is mounted inside a camera module package, which protects the image sensor against damage from environmental hazards that may arise after the image sensor chip has been packaged. The image sensor chip commonly is electrically connected to the leads of the camera module package through bond wires that are attached to bond pads on the topside of the image sensor chip or through solder bumps on the backside of the image sensor chip. Although solder-bump bonded electrical connections provide improved data rate performance due to their shorter length, wirebonded electrical connections are favored in terms of cost and throughput.
In addition to post-packaging-generated contamination, the performance and the yield of image sensors also are compromised by the presence of contaminants, such a dust and other particles, that are generated during fabrication, dicing, and packaging of the image sensors. The presence of these contaminants tend to damage various components of the image sensors, including the pixels in the image (or pixel) areas, the bond pads, and the electrical traces on the surfaces of the image sensors that carry signals to and from bonding pads.
Various approaches have been proposed for protecting wire-bondable image sensors against damage and defects that are caused by contaminants that are generated prior to the dicing and packaging of individual image sensor chips. In one approach, a photo-etchable cover wafer is attached to a semiconductor wafer containing a plurality of image sensor dice prior to dicing and packaging of individual image sensor chips. The cover wafer protects the image sensor substrate from environmental hazards, such as particulate contaminants, moisture, processing agents such as solvents, and inadvertent scratching of the image sensor substrate. In another approach, a thin, transparent, water-repellant and oil-repellant resin is applied over the image sensors prior to dicing and packaging. The low surface energy of the resin layer is purported to substantially prevent dust from attaching to the resin layer and to readily allow any dust that manages to attach to the resin layer to be removed easily by means of a cotton swab or the like.
The proposed image sensor protection approaches described above appear to reduce the damage caused by contaminants during fabrication and packaging of the image sensors. These approaches, however, do not address the additional need to reduce the adverse impact caused by the presence of contaminants that interfere with the reception of light by the pixels in the imaging area and thereby cause defects in the images captured by the image sensors.